Hola konzo, as stated in the Virtual box documentation (http://www.virtualbox.org/manual/ch03.html):
Note
Enabling the I/O APIC is required for 64-bit guest operating systems, especially Windows Vista; it is also required if you want to use more than one virtual CPU in a virtual machine.
You can disable after install, but this has no effect. But there are workarounds if you google for it to disable APIC, as keeping this setting enabled may affect performance.